How can I make a PSI pattern that matches a char in a token?

I want to create a completion contributor that provides completions after < char in a “plain text” token.

This is for MiniMessage plugin which has very lenient rules for syntax errors and therefore allows you to write such: <red>Hello World!<. This would parse to the <red> tag having it’s own class holding three tokens and a PlainText class holding just the one PLAIN_TEXT token.

How can I make an element pattern for a completion contributor? Or alternatively how can I write a completion contributor providing suggestions after < char?